The present invention relates to a submarine branching unit of an optical submarine cable communication system. More particularly, the present invention relates to a submarine branching unit having an asymmetrical architecture with a switching system designed to enhance the life of relay contacts by switching at zero current conditions.
Underwater or submarine cable systems were originally designed to provide a telecommunications link between two landing points separated by a body of water, such as between France and England. Optical fiber within the submarine cable could carry high bandwidth telecommunications across tens of kilometers without the need for amplification or regeneration.
As applications evolved that required transmission across larger distances, optical repeaters in the form of regenerators or amplifiers were required within the cable span, which mandated the availability of electrical power. Power feed lines and optical transmission lines were provided together in the submarine cables. These cable systems were soon improved to provide a telecommunications connection to three or more separate landing points by employing a branching unit at the junction of multiple cables under the body of water. A standard branching unit connected a main cable from one landing point and two spur cables from second and third landing points, respectively, in a Y-shaped arrangement.
FIG. 1 illustrates a typical three landing point connection scheme. Branching unit 100 physically interconnects the cables, coordinates the routing of the cables, and provides for power switching between the cables, among other things. Conventionally, stations at the landing points power repeaters 120 within the three cables in a two-sided and one-sided arrangement. In particular, the transmission of electrical direct current along a power feed line of the intervening cables is established between any two of the landing points, and the third landing point passes electrical current through its respective cable to a ground at sea earth. Relays within the branching unit establish the two-sided (bilateral) and one-sided (unilateral) feeding configuration and help to change it if a fault arises in one of the branches. More elaborate schemes also exist for connecting multiple landing points and multiple branching units in a submarine network.
The act of configuring a branching unit refers to coordinating the application of power from the respective landing points by providing a first power connection (two-sided) between two landing points and a second power connection (one-sided) between the third landing point and sea earth. The two-sided power connection between the above two landing points can be either a two-end feeding as shown in FIGS. 2A, 2B, i.e., a generator is present at each of the two landing points or, alternatively, a one-end feeding, i.e., a generator is present at one of the landing points and the cable is grounded at the other of the two landing points.
FIGS. 2A and 2B illustrate the two configuration steps. As shown in FIG. 2A, a low current is caused to flow from landing point B to landing point C via branching unit 10. In this arrangement, the line from B to C is nominated as the main trunk, and the spur branch to landing point A is left in an open-circuit condition. FIG. 2B shows that after the main trunk has been established, the one-sided powering of the spur takes place by shunting the spur to sea earth at the branching unit and feeding the spur from landing point A. For a branching unit, the two-sided power connection may be established between any two of the three landing points.
Various publications describe this configuration process for symmetrical branching units. U.S. Pat. No. 5,196,984 discloses a branching unit which employs electrical power feeding for repeaters and multiple branching units. The branching unit terminates three line cables and a sea earth and includes three high voltage element relays, only one of which is energized at a time. When electrical power is supplied between any two line cables, the third is isolated and connected to the sea earth. Short circuit or open circuit faults in one line cable may be isolated and connected to the sea earth, while allowing powering of the remaining two line cables that are free of faults.
Similarly, U.S. Pat. No. 5,214,312 discloses a power feed line switching circuit for a submarine branching unit having first, second and third electrical paths connected in a Y-shaped connection, and first, second and third terminals connected respectively thereto. The power feed line switching circuit also has first, second and third relays each including a drive unit inserted in the first, second and third terminals and a switching unit for disconnecting the corresponding terminal and connecting the terminal to the ground. The relays control the connections between the terminals and electrical paths for establishing a one-end power feed line or a two-end power feed line to maintain power feed for repeaters and the submarine branching unit by the plurality of relays.
U.S. Pat. No. 5,644,466 discloses a submarine cable branching system including a plurality of directional relays in respective power feed paths branched at a common node. Each of the directional relays is energized by a current flowing through the power feed path in a predetermined direction for energizing a corresponding switch provided in a different power feed path to establish a bilateral feed path and further a unilateral feed path. A bypassing switch is provided across one of the switches that forms the bilateral feed path such that the bypassing switch is urged to close in response to energization of a self-sustaining relay provided in the unilateral feed path.
Since the branching unit, repeaters and cables are laid underwater and are thus difficult to maintain, it is important that they have a high reliability to ensure uninterrupted telecommunications between the landing points. During power-up of the main trunk, however, the spur cable will acquire a charge related to the voltage drop at the branching unit and the length of the spur. Switching the spur cable to sea earth (typically the sea water) can cause a rapid discharge, which may damage contacts and relays with the branching unit. In particular, a relay that causes the spur cable to contact the sea earth is typically a high-voltage changeover relay. When actuation of the relay causes a make of the switch, an arc discharge will liberate a quantity of contact material. Acting as charge carriers, the liberated material may pass between the just opened contacts in the three-connection relay. An avalanche effect could take place that causes the main cable to discharge across the contact gap with large amperage, leading to imminent failure of the relay and connection of the main cable to sea earth. In this scenario, either it could be impossible to power the system up and configure the system or an oscillation could be initiated between the main trunk and spur (see, e.g., U.S. Pat. No. 5,517,383 at columns 1 and 6). Consequently, it is advantageous to avoid current arcing across the relay contacts.
U.S. Pat. Nos. 5,517,383 and 5,841,205 propose circuits to avoid this damage. In the '383 patent, an additional relay D and switch D1 are used to avoid dangerous arcing in the high voltage changeover relay such as C. Relay D is not a changeover relay and can withstand a greater amount of arcing without failing.
The '205 patent discloses the use of a two-stage relay to avoid damage from arcing. In this arrangement, the spur is disconnected from the main trunk when a defined current level is reached and then discharged by shorting its terminal station end (i.e., not underwater). After current in the main trunk ramps to a second level, the spur is connected to sea earth at the branching unit without arcing.
U.S. Pat. No. 4,798,969 discloses a branching unit circuit that includes an auxiliary relay that prevents the spur branch from re-connecting to the main trunk when its current falls to zero. A relay K1 in the main trunk controls the switch connecting the spur branch to sea earth. A latch-up relay K4 in the spur branch holds the spur branch out of contact with the main trunk if relay K1 opens and relay K4 remains energized.
U.S. Pat. No. 5,334,879 discloses a feeding system for a submarine cable communications system. The system includes a plurality of branching units provided on the communication cable and terminal stations each being connected through a repeater to one of the branching units. Each branching unit contains at least one switching circuit for switching to connect at least one of the repeaters to the communication cable or to ground depending on the current conducting through the feedline in the cable. The current thresholds of the switching circuits in the branching units are preferably different from each other.
U.S. Pat. No. 5,491,368 shows a power feed path switching circuit for coupling a first, second and third station in a normal state. The first unit senses a first current flowing in a first path connecting the first and second stations and disconnects a second path connecting the third station when the first current is sensed. A second unit senses a second current flowing in the first path and discharges, gradually, the second path. A third unit senses current flow in the first path and grounds the second path when the third current is sensed.
WO 97/06610 discloses a branching unit for an optical fiber system that is not symmetrical. The branching unit has first and second connections for coupling two sections of trunk cable and a third connection for an optical fiber spur cable. A switching system is provided responsive to a control voltage placed on the spur cable to apply an earth ground to the conductor of the trunk cable. This is used to maintain a current path in one trunk section when a failure or break occurs in the other trunk section.
U.S. Pat. No. 5,790,358 describes a feeding pass switching circuit. The switching circuit includes three terminals respectively connected to a feeding pass. The switching circuit is provided with a relay connected between a first terminal and a second terminal and activated by a current flowing in one direction. Another relay is connected between the second terminal and a third terminal and activated by a current flowing in either direction. One more relay is connected between the first terminal and the third terminal and activated by a current flowing in one direction.
Applicants have discovered that conventional circuits for power feed equipment in submarine branching units do not insure continued operation of good feed paths following the failure of a faulted feed path. Applicants have also discovered that conventional circuits do not provide adequate protection of relay contacts against possible arcing damage due to high current pulses as a consequence of cable discharge.